There has been a need for a high-temperature resistant magnetic tape material for use in such applications as aircraft flight-recorder systems and other applications wherein very high temperature resistance is required. Heretofor, such systems have been made from metal rather than polymeric-based tapes. The metal tapes, besides being expensive to manufacture, have not been entirely satisfactory from an operational point of view, either.
There have been intensive and extensive efforts to develop a spooled magnetic tape which will remain magnetically readable at temperatures of 400.degree. F. to 800.degree. F. These efforts have been failures. For example, the tape disclosed by Beck et al. in U.S. Pat. No. 3,484,286 utilizes a substrate that is resistant to thermal degradation and a magnetic coating thereover comprising a matrix copolymer of vinylidene fluoride and hexafluoropropylene and a magnesia stabilizer. Beck et al. disclose nothing very relevant to solving the substantial problems encountered in use of spooled tape at high temperatures which problems have to do with inter-adhesion of adjacent layers and loss of adhesion to substrate, etc. Beck et al. do disclose what has proved to be a very useful substrate material for such applications, i.e. the polyimide substrate sold under the trade designation Kapton by E. I. DuPont de Nemours and Company. However, when used in spooled tape, this material has always presented an extraordinary sticking or clinging tendency creating a far greater problem than any associated with the more commonly-used polyethylene terephthalate films. Another useful polymer substrate having similar properties is a poly (para-phenyleneterephthalamide) sold under the trade designation Kelvar by DuPont. For the purpose of this application, such polymers will be characterized as "refractory polymers," i.e. polymers that retain their structural properties above 450.degree. F.
In art heretofore unrelated to the Beck et al. patent, certain prepolymerized polyurethane resins have been found to be useful in magnetic tape formulations. For example. U.S. Pat. No. 3,649,541 to Ingersoll describes some such prepolymerized polyurethane resins. Some of these are further described in U.S. Pat. No. 2,899,411 to Schollenberger, U.S. Pat. No. 2,871,218 to Schollenberger, U.S. Pat. No. 3,384,679 to Stetz and U.S. Pat. No. 3,320,090 to Graubart also discuss such prepolymerized polyurethane systems which, for a number of reasons, seem to be more desirable for use in magnetic coatings than reactive systems formed.
In further art, heretofore, unrelated, it has been known to use conductive carbon black on the backing tape -- usually for antistatic purposes on high-speed tapes. U.S. Pat. No. 3,293,066 to Haines discloses use of an acetylene black in a resin binder for such a purpose.
No combination of the above art has suggested a means for solving the problem of substituting a heat-resistant magnetic tape for the metallic tapes now used in applications wherein very high temperatures are encountered.